Cryptography Reference
In-Depth Information
Figure 9.9: The main window of
st-wav.exe
, the S-Tools program for
hiding information in a WAV file. The program displays the changed
parts of the waveform in red, but this detail is lost in a black-and-
white book.
cause. This random process guarantees that the distortion is dis-
tributed throughout the file so it is not so apparent. It also makes
it difficult for some attacker to figure out which bits are important.
S-Tools uses the MD5 algorithm to ensure that the random num-
bers are cryptographically secure. A more modern hash function like
SHA256 may make more sense now that researchers are discovering
a number of weaknesses in MD5 .
In fact, a random subset can have some other uses. First, if a
person selects a small, random subset to store information, then an-
other person could do the same thing. If both use different sources
of randomness to choose small subsets, then there is a good chance
that very few bits will end up in both subsets. Error-correcting codes
can help recover from this overlap. This could allow several people to
use the same file to pass information to several different other ones.
Steve Walton suggested this approach in his article, “Image Au-
thentication for a Slippery New Age” [Wal95b]. This approach uses
a general, two-dimensional random walk that weaves around a pic-
ture. Occasionally, the path may wrap around itself which requires
keeping track of where the path has been before. Hide and Seek,
in contrast, views the picture as a one-dimensional list of pixels and
chooses a random number of pixels to jump ahead.
